platform2: Replace NULL with nullptr in minor components

Replace NULL with nullptr in C++11-enabled components of platform2
that don't have too many occurrences of NULL. Other major
"offenders" will be handled separately.

BUG=None
TEST=FEATURES=test emerge-link app-shell-launcher buffet easy-unlock platform2 metrics wimax_manager

Change-Id: I61b25a057e3d6865908bc74f9f3d4cb55e08af26
Reviewed-on: https://chromium-review.googlesource.com/214837


Tested-by: Alex Vakulenko <avakulenko@chromium.org>
Reviewed-by: Ben Chan <benchan@chromium.org>
Commit-Queue: Alex Vakulenko <avakulenko@chromium.org>